Optical fiber polarization scrambler and operating parameter input method therefor

ABSTRACT

An optical fiber polarization scrambler decreasing the degree of polarization (DOP) of light efficiently using at least two optical fiber birefringence modulators ( 10, 11 ) and an operating parameter input method for the same. Birefringence modulators are constructed with hollow cylindrical piezo devices ( 10, 11 ) and a strand of optical fiber ( 30 ) wound continuously on the outer walls of the piezo devices ( 10, 11 ). An angle between the birefringence modulators consisting of the optical fiber polarization scrambler is configured to compensate the effect of circular birefringence. A method for exactly configuring birefringence modulation frequency and birefringence modulation amplitude to achieve effective polarization scrambling for the optical fiber polarization scrambler is also provided.

TECHNICAL FIELD

The present invention relates to a polarization scrambler, morespecifically to an optical fiber polarization scrambler whicheffectively decreases degree of polarization of the output light usingat least two optical fiber birefringence modulators.

The present invention also relates to an operating parameter inputmethod of an optical fiber polarization scrambler, more specifically toa method for inputting a predetermined values of parameters such as amodulation frequencies and modulation amplitudes established byexperiments to effectively operate the optical fiber polarizationscrambler.

BACKGROUND ART

Polarization scramblers are devices that converts highly polarized lightinto light with a scrambled polarization by modulating the state ofpolarization(“SOP”). Degree of polarization(“DOP”) represents the ratioof polarized light to total light. For example, the DOP of perfectlypolarized light is 100%, whereas the DOP of completely depolarized lightis 0%. The function of the polarization scrambler is to decrease the DOPin time average. An ideal polarization scrambler forces input light of100% DOP to output light of 0% DOP.

When light goes through an optical component having apolarization-dependent loss, output power depends on the input SOP ofthe light. In this case, constant time-averaged output power can beobtained regardless of the input SOP by inserting a polarizationscrambler in front of the optical component. Enhanced signal-to-noiseratio can also be obtained with the polarization scrambler in afiber-optic sensor, optical measurement system and long-haul opticaltransmission system.

The output SOP is evolved by a birefringence medium, especially theamount of the birefringence and the angle of the birefringence axis whenpolarized input light is propagating along the optical fiber. For thelight which has been transmitted along a long optical fiber over a fewmeters, however, the output SOP is apt to change since the birefringenceof the fiber is easily affected by small environmental perturbation suchas temperature and pressure. This leads to output power fluctuation ofthe light which went through an optical device having certainpolarization-dependent loss. However, the fluctuation of the outputlight could be prevented if it is averaged in time by decreasing the DOPof the output light through polarization modulation.

If the polarization direction of the input light coincides with thedirection of the birefringence axis of the birefringence modulator, theoutput SOP will not vary even with any change of the birefringence.Therefore, in order to induce polarization modulation of the outputlight regardless of the input SOP using birefringence amplitudemodulation, at least two polarization modulators whose birefringenceaxes form an angle of 45° must be used.

In the prior art, a polarization scrambler is implemented by inducingbirefringence modulation in an integrated optical circuit, such as alithium niobate (LiNbO₃) optical waveguide. However, this kind of thepolarization scrambler has the disadvantages of low efficiency,difficulty in connecting two or more birefringence modulators, and highinsertion loss.

U.S. Pat. No. 4,923,290 discloses a basic concept of the polarizationscrambler using an optical fiber. However, since the method of directlyexerting pressure onto the optical fiber is used to implement thescrambler, the device for inducing the birefringence has a lowefficiency with scrambling frequency lower than few hundred Hz, which istoo low. Also the birefringence axis is subject to easy change and theoutput SOP deteriorates. This renders the realization of thepolarization scrambler with good performance difficult.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an optical fiberpolarization scrambler which can efficiently carry out polarizationmodulation and make the time-averaged DOP zero regardless of the inputSOP and an operating parameter input method therefor.

Another object of the present invention is to provide an optical fiberpolarization scrambler with very low insertion loss and stableperformance regardless of the input SOP using enhanced optical fiberbirefringence modulators whose modulation frequencies are in orders ofhundred kHz to MHz and an operating parameter input method therefor.

A further object of the present invention is to provide an optical fiberpolarization scrambler comprising a strand of optical fiber whosestructure can counterbalance the effect of the circular birefringencecaused by twisting the optical fiber between the neighboringbirefringence modulators, while providing the most efficient operatingparameter input method therefor.

In order to achieve the above-mentioned objects, the optical fiberpolarization scrambler according to the present invention comprisesoptical fiber birefringence modulators and alternating voltage sources.The optical fiber birefringence modulators include at least two hollowcylindrical piezo devices and a continuous optical fiber wound aroundouter walls of the piezo devices. The alternating voltage sourcesrespectively apply alternating voltage to each optical fiberbirefringence modulator to induce birefringence modulation. Thecontinuous optical fiber is wound around the piezo devices without anytwisting in axial direction of the optical fiber to prevent inducing ofcircular birefringence thereat. But the section of the optical fiberlinking the neighboring birefringence modulators is twisted so that theinput light passed the front birefringence modulator with itspolarization plane being parallel to a principal axis of the frontbirefringence modulator can be launched into the back birefringencemodulator with its polarization plane forming an angle of 45±90n degreeswith respect to a principal axis of the back birefringence modulator,where n is an integer.

According to the present invention, the hollow cylindrical piezo devicescan have different wall thickness to make the resonance frequency of thethickness mode of each piezo device of the optical fiber birefringencemodulators different.

Also, it is preferable that the means for compensating effect ofcircular birefringence caused by twisting applies further twist to thesection of the optical fiber linking neighboring birefringencemodulators. Additional twist is of the angle corresponding to 8% of(45±90n) degrees and the total angle between the principal axes of theneighboring birefringence modulators becomes 1.08×(45±90n) degrees whichalso means the total twist of the linking optical fiber, where n is aninteger.

Preferably, the optical fiber is a single-mode optical fiber whoseintrinsic birefringence is 5×10⁻⁶ or less.

Also, preferably, the linear birefringence induced by tension windingand bending of the optical fiber on the outer wall of the piezo deviceis maintained 30 times or more than the intrinsic birefringence of theoptical fiber, and more preferably the wound optical fiber is annealedto eliminate spurious birefringence.

In addition, the cross-section of the optical fiber is in a shape of D,and the optical fiber can be wound with the flat plane of the opticalfiber abutting the outer walls of the piezo devices to prevent twist ofthe optical fiber in the axial direction.

On the other hand, the optical fiber polarization scrambler according tothe present invention can further comprise means for measuringtemperature of the birefringence modulator; and means for determiningmodulation amplitude corresponding to the temperature sensed by themeans for measuring the temperature.

In another structure of optical fiber polarization scrambler accordingto the present invention, the optical fiber polarization scramblercomprises: optical fiber birefringence modulators including at least twohollow cylindrical piezo devices and an optical fiber wound around outerwalls of the hollow cylindrical piezo devices; and alternating voltagesources applying alternating voltage to each optical fiber birefringencemodulator to induce birefringence modulation, wherein the polarizationcontroller is inserted between the neighboring birefringence modulators.

Also in this structure, the hollow cylindrical piezo devices can havedifferent wall thickness to make the resonance frequency of thethickness mode of each piezo device of the optical fiber birefringencemodulators different.

Preferably, the polarization controller is an all-fiber polarizationcontroller.

In addition, this structure can further comprise means for measuringtemperature of the birefringence modulator; means for determiningmodulation amplitude corresponding to the temperature sensed by themeans for measuring the temperature; and means for maintainingtemperature of the birefringence modulator constant.

The operating parameter input method for an optical fiber polarizationscrambler, according to the present invention, comprising optical fiberbirefringence modulators including at least two hollow cylindrical piezodevices and a continuous optical fiber wound around each outer wall ofthe piezo devices without twist in axial direction of the optical fiberto prevent inducing of circular birefringence; and alternating voltagesources for applying alternating voltage to each optical fiber modulatorto induce birefringence modulation is as follows.

The alternating voltages applied to birefringence modulators havesinusoidal waveforms whose frequency difference is larger than therequired bandwidth, the amplitude of the alternating voltage beingconfigured as a value minimizing the DOP measured from output lightsignal sensed by an optical sensor after light from the light sourcepassing in turn an input polarization controller, the optical fiberpolarization scrambler, an output polarization controller and apolarizer.

Preferably, the resonance frequency corresponding to the thickness modeof the piezo device is used as the birefringence modulation frequency.

On the other hand, the birefringence modulation amplitude of eachoptical fiber birefringence modulator can be selected from solutions ofthe zeroth-order Bessel function to operate the optical fiberpolarization scrambler.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will be more clearlyunderstood to those skilled in the art with reference to theaccompanying drawings in which:

FIG. 1 shows a basic structure of the optical fiber birefringencemodulator which is the gist device in constructing a polarizationscrambler according to the present invention;

FIG. 2 is a graph showing the measured result of impedance as a functionof the frequency for a hollow cylindrical piezo device used in anembodiment of the present invention;

FIG. 3 shows the structure of the device for analyzing thecharacteristics of a single stage polarization scrambler;

FIG. 4 is a graph showing the temperature dependency of the requiredvoltage according to the operating frequency of the single stagepolarization scrambler;

FIG. 5 is a graph showing a change in degree of polarization as afunction of the temperature for the single stage polarization scrambler;

FIG. 6 is a graph showing the measured impedance in the proximity ofresonance frequency corresponding to thickness mode of the hollowcylindrical piezo device of a different size;

FIG. 7 is a graph showing the measured result of voltage needed tooperate the polarization scrambler for a few frequencies between theresonance frequency and accompanying anti-resonance frequency as afunction of the temperature;

FIG. 8 shows a basic structure of a double stage polarization scramblerconsisting of two birefringence modulators; and

FIG. 9 is a graph showing the angle between the birefringence modulatorsneeded for operating the polarization scrambler regardless of the inputSOP.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in detail referring to theaccompanying drawings. However, the embodiments hereinafter describedshould be interpreted as illustrative and in no sense limiting.

FIG. 1 shows a basic structure of the optical fiber birefringencemodulator which is the gist device in constructing a polarizationscrambler according to the present invention. Referring to FIG. 1, asingle mode optical fiber 30 is wound around an outer wall of acylindrical piezo device 10 such as a piezoelectric transducer(“PZT”)without any twist in axial direction of the optical fiber 30 to preventinducing of circular birefringence. In order to prevent the twist, anoptical fiber 30 can have a cross-section in a shape of “D” with theoptical fiber 30 wound around the outer wall of the piezo device 10 withthe flat plane abutting the outer wall of the piezo device 10. When acertain voltage signal of sinusoidal waveform is applied to the piezodevice 30 by an alternating voltage source 40, not only the phasemodulation is induced, but also the birefringence modulation is inducedfor the light passing through the optical fiber 30. Direction parallelto the axis of the cylinder 21 and direction normal to this, that isdirection 22 normal to the cylindrical surface, become birefringenceaxes, where the amplitude of the birefringence modulation is denoted asthe following equation (1).

φ_(m) =V _(m)α sin(ω_(m) t)  (1)

where V_(m) and ω_(m) each corresponds to the amplitude and the angularfrequency of the voltage signal applied to the piezo device, and αdenotes the birefringence modulation coefficient. The coefficientdepends not only upon jacket material and the length of optical fiber,but also on the frequency of the applied voltage signal.

The above device has been used only as a phase modulator because thebirefringence modulation is extremely low to be used at the range offrequency less than 100 kHz. However, the method according to thepresent invention carries out the birefringence modulation in theproximity of the resonance frequency corresponding to the wall thicknessof the cylindrical piezo device to enhance the efficiency for theapplied voltage of the birefringence modulator. The resonance frequencydepends on the raw material and wall thickness of piezo device, butgenerally it is denoted as the following equation (2).

f·T=200 [kHz·cm]  (2)

Therefore, when using a hollow cylindrical piezo device whose wallthickness T is about 1 mm to 5 mm, a resonance frequency f of 400 kHz to2 MHz can be used as the birefringence modulation frequency.

On the other hand, input light has to be linearly polarized, and thepolarization plane must be adjusted to 45° with respect to thebirefringence axes 21 and 22 for the birefringence modulator of FIG. 1to operate as a polarization scrambler. For linearly polarized inputlight with polarization plane tilted 45 degrees with respect to thebirefringence axis of the birefringence modulator modulating as inequation (1), the following equation (3) has to be satisfied for anoutput light to have a DOP of 0 when averaged in time sufficientlylonger than the modulation period.

J ₀(V _(m)α)=0  (3)

Hear, the J₀ denotes the zeroth-order Bessel function of the first kind.Therefore, the birefringence modulation amplitude V_(m)α has to be asolution of the zeroth-order Bessel function, such as 2.405, 5.520, . .. That is, a frequency with large birefringence modulation coefficientis set as the frequency of the voltage signal and the modulationamplitude is set so that the V_(m) α is a value such as 2.405, 5.520 orthe like.

On the other hand, when a single mode optical fiber is wound around thepiezo device, it is important to prevent the optical fiber from twistingso that only linear birefringence should be induced in the opticalfiber. To achieve this, in the present invention, a spun optical fiberis used as the single mode optical fiber which has very low intrinsicbirefringence. Furthermore, the optical fiber is tension-wound andannealed at the temperature of about 100° C. so that the linearbirefringence axis is parallel to the axis of the piezo device.

The tension-winding was carried out under the condition where the linearbirefringence induced by the bending of the optical fiber caused by theradius of curvature of the piezo device and~the tension in the opticalfiber when winding around the piezo device is maintained 30 times ormore than the intrinsic birefringence of the optical fiber.

FIG .2 is a graph showing the measured result of the impedance as afunction of the frequency for the hollow cylindrical piezodevice(diameter: 1.8″, thickness: 0.1″, length: 0.9″) used in theembodiment according to the present invention. Referring to FIG. 2, itcan be seen that the impedance is lowest in the proximity of theresonance frequency 45 of about 770 MHz, and that the impedance haslocal maximum in the proximity of the anti-resonance frequency 46 ofabout 875 kHz.

A single stage polarization scrambler was constructed with the piezodevice having the impedance characteristics of FIG. 2 and the spunoptical fiber was wound about 40 turns around the piezo device with thetension of 30 kpsi.

FIG. 3 shows the experimental setup for analyzing the characteristics ofthe single stage polarization scrambler. A light source 51 is a laserdiode emitting polarized light at the wavelength of 1.55 μm. An inputpolarization controller 52 is for controlling the polarization state ofthe input light to produce linearly polarized light. If linearlypolarized input light parallel or perpendicular to the principal axis ofthe birefringence modulator is entered, there will be no effect of thepolarization modulation. In order to maximize the effect of thepolarization modulation, linearly polarized light whose polarizationplane is aligned to form an angle of 45 degrees with respect to theprincipal axis should be entered into the birefringence modulator. TheDOP of the output light is calculated by using an oscilloscope 56 afterconverting the light passing the output polarization controller 53 andthe polarizer 54 into electric signals and then measuring the DCcomponent of it. Here, the DOP is the ratio of the maximum change to theaverage in the amplitude of the DC component while varying the outputpolarization controller 53. Therefore, an optimum voltage amplitude canbe determined by adjusting the amplitude of the sinusoidal voltageapplied to the birefringence modulator so that the DOP of output lightis minimized. Using this method, the optimum operating parameter of theoptical fiber polarization scrambler can be obtained.

Next, FIG. 4 shows the temperature dependency of the required sinusoidalvoltage amplitude for a few frequencies of 800.8, 808.6, 835.9, 855.5,871.1 and 890.6 kHz which are in the proximity of the resonancefrequency (about 770 kHz) and the anti-resonance frequency (about 875kHz). Referring to FIG. 4, it can be seen that the amplitude of thevoltage signal became lower when approaching the resonance frequencywhereas increased when approaching the anti-resonance frequency. Itshould be noted that the amount of the birefringence modulation is toosmall outside the frequency of this range to be useful. Also, it couldbe seen that the amplitude of the voltage signal should be decreasedwhen the environmental temperature of the birefringence modulator israised. Furthermore, it should be pointed out that temperature stabilityis enhanced at the resonance frequency.

Therefore, from the result shown in FIG. 4, when inputting operatingparameter of the optical fiber polarization scrambler, it could be seenthat using the thickness mode resonance frequency of the piezo device asthe birefringence modulation frequency is preferable.

FIG. 5 is a graph showing the change in the degree of polarizationagainst changes in the temperature of the polarization scrambler. Inorder to obtain the result of FIG. 5, the degree of polarization ismeasured while changing the temperature of the birefringence modulatorafter adjusting the amplitude of the voltage signal at each frequency tominimize the degree of polarization under the room temperature. As canbe seen in FIG. 4, when applied voltage is constant, the degree ofpolarization increases since the amount of birefringence modulation islarger than 2.405 as the temperature rises. In order to maintain thedegree of polarization within 10% at the temperature ranging from 0 to60° C., the amplitude of the applied voltage signal has to becompensated for the temperature. By approximating linearly andcompensating for the temperature, a low degree of polarization was to bemaintained for a wide range of temperature.

On the other hand, when the compensation for the temperature is notcarried out, operation at the resonance frequency will maintain a lowerdegree of polarization for a wider range of temperature compared withwhen operated at the anti-resonance frequency. The temperaturecompensation can be realized through temperature measuring means formeasuring the temperature of the birefringence modulator and means fordetermining the modulation amplitude corresponding to the temperaturesensed by the temperature measuring means.

The degradation of the DOP caused by temperature change can be preventedby using temperature maintaining means which maintains the temperatureof the birefringence modulator constant.

FIG. 6 shows a measured result of the impedance in the proximity of theresonance frequency of the thickness mode of the hollow cylindricalpiezo device(diameter: 1″, thickness: 0.06″, length 0.5″) of a differentsize.

FIG. 7 is a measured result of the required voltage to operate thepolarization scrambler for several frequencies of 1.2, 1.25, 1.3, 1.35,1.4, 1.45, 1.5, 1.55 and 1.6 MHz against the temperature. As can be seenhere too, the amplitude change of birefringence modulation caused by thetemperature can be prevented when the polarization scrambler is operatedat a resonance frequency of 1.25 MHz. The resonance frequency isdetermined by the wall thickness of the cylindrical piezo device. Usingthis characteristics, polarization scrambler stable against the changeof the temperature even without temperature compensation and having highelectric efficiency can be manufactured.

On the other hand, since the performance of the single stagepolarization scrambler varies extremely depending on the input SOP, twoor more birefringence modulators are needed to manufacture apolarization scrambler which is not affected by the change of the inputSOP. The birefringence axes of each birefringence modulator must form anangle of 45 degrees. That is, since a first birefringence modulator doesnot produce polarization modulation for the input light whosepolarization plane is parallel to the birefringence axis of the firstbirefringence modulator, the effect of polarization scrambling isachieved after passing through a second birefringence modulator whosebirefringence axis forms an angle of 45 degrees with respect to thelinearly polarized light.

In order to maintain the degree of polarization to zero regardless ofthe input SOP when birefringence modulation is carried out by thesinusoidal voltage signal, the amplitude of each birefringencemodulation must satisfy the following equation 4.

J ₀(V _(m1)α₁)=J ₀(V _(m2)α₂)=0  (4)

Hear, J₀ is the zeroth-order Bessel function, and birefringencemodulation amplitudes V_(m1)α₁ and V_(m2)α₂ must be solutions of thezeroth-order Bessel function such as 2.405, 5.520, . . .

While the frequencies of the voltage signals are selected from theresonance frequency corresponding to the thickness mode, the differenceof the two frequencies should be large enough but sufficiently smallerthan a required bandwidth and the amplitude V_(m)α should have suchvalues as 2.405, 5.520, . . . For instance, if the polarizationscrambler is used for suppressing so called polarization-dependent gainof an erbium-doped fiber amplifier, the frequency difference should belarger than a few tens of IcHz because the gain recovery ratecorresponding to the erbium-doped fiber amplifier is of a few kHz.

FIG. 8 shows a basic structure of the double stage polarizationscrambler consisting of two birefringence modulators. The optical fiberconnecting the two birefringence modulators is continuous and the angletherebetween is adjusted by twisting the optical fiber. On the otherhand, when the optical fiber is twisted, circular birefringence isinduced in the optical fiber and the polarization plane is rotated about8% of the twist angle between 22 and 24. Of course, there must be notwisting of the optical fiber in axial-direction of the optical fiber atthe optical fiber wound at each birefringence modulator.

Therefore, in an optical fiber polarization scrambler whose opticalfiber is continuously wound (on neighboring hollow cylindrical piezodevices, the twist between the two birefringence modulators must becarried out with the angle for the compensation of the effect ofrotation of the polarization plane caused by circular birefringence, inorder to input at 45 degrees with respect to the principal axis of thesecond birefringence modulator when the polarization plane of the inputlight is parallel to the principal axis of the first birefringencemodulator.

That is, in order to input the polarization plane parallel to thebirefringence axis of the first birefringence modulator at 45 degreeswith respect to that of the second birefringence modulator, the twobirefringence modulators should form an angle of approximately 49degrees (45+45×0.08).

FIG. 9 shows analyzed result of this condition, where the angle betweenthe birefringence modulators (the angle between 22 and 24 of

FIG. 8) necessary for the operation of the polarization scramblerregardless of the polarization state of input light is measured. Theresult can be expressed as a linear function of the following equation(5).

Y=X+0.08X=1.08X  (5)

where, X is 45±90n which is the preferred angle in general. But Y is anactual angle between the two neighboring birefringence modulators, thatis, twist angle of the optical fiber which can be any value among ±49,±146, ±243, ±340 . . . Especially, the angle of ±340 degrees can enablea polarization scrambler of an overall compact size since the height ofthe two birefringence modulators under the twisted state is reduced.

Also, the angle between the neighboring birefringence modulators can beconfigured arbitrary by placing an additional polarization controllerbetween the two neighboring birefringence modulators. Preferably, theall-fiber polarization controller disclosed in Korean patent applicationNo.97-20056 can be used as the polarization controller.

As described above, the optical fiber polarization scrambler of thepresent invention utilizes the resonance frequency of the thickness modeof the cylindrical piezo device to efficiently induce birefringencemodulation and to be stable against temperature change.

The present invention provides an optical fiber polarization scramblerwith a structure which can compensate the effect of circularbirefringence caused by twisting the optical fiber between theneighboring birefringence modulators consisting the optical fiberpolarization scrambler, and an operating parameter input method forefficiently operating the same.

What is claimed is:
 1. An optical fiber polarization scrambler having optical fiber birefringence modulators and alternating voltage sources, said optical fiber birefringence modulators including at least two hollow cylindrical piezo devices and a strand of optical fiber wound around outer walls of the piezo devices, said alternating voltage sources respectively applying alternating voltage to each optical fiber birefringence modulator to induce polarization modulation, wherein the optical fiber wound around the piezo devices is not twisted in axial direction of the optical fiber to prevent inducing of circular birefringence thereat, said optical fiber polarization scrambler comprising means for compensating effect of circular birefringence caused by twist between the neighboring birefringence modulators in order to input the input light with its polarization plane forming an angle of 45±90n degrees with respect to the direction of a principal axis of a back birefringence modulator when the polarization plane of the input light is parallel to the principal axis of the front birefringence modulator, where n is an integer.
 2. The optical fiber polarization scrambler of claim 1, wherein the hollow cylindrical piezo devices have different wall thickness to make the resonance frequency of the thickness mode of each piezo device of the optical fiber birefringence modulators different.
 3. The optical fiber polarization scrambler of claim 1, wherein an optical fiber constituting the optical fiber birefringence modulators don't have any splicing point between the two optical fiber birefringence modulators.
 4. The optical fiber polarization scrambler of claim 1, wherein the means for compensating effect of circular birefringence applies further twist to the neighboring birefringence modulator at an angle corresponding to 8% of the angles of 45±90n degrees, where n is an integer.
 5. The optical fiber polarization scrambler of claim 1, wherein the optical fiber is a single mode optical fiber whose intrinsic linear birefringence is less than 5×10⁻⁶.
 6. The optical fiber polarization scrambler of any one of claims 1-5, wherein the linear birefringence induced by tension-winding the optical fiber caused by radius of curvature of the outer wall of the piezo device is maintained 30 times or more than the intrinsic birefringence of the optical fiber, and the wound optical fiber being heat-treated to quench spurious birefringence.
 7. The optical fiber polarization scrambler of claim 1, wherein the cross-section of the optical fiber is in a shape of D, and the optical fiber is wound with the flat plane of the optical fiber abutting the outer walls of the piezo devices in order to prevent twist of the optical fiber in the axial direction.
 8. The optical fiber polarization scrambler of claim 1, further comprising: means for measuring temperature of the birefringence modulator; and means for determining modulation amplitude corresponding to the temperature sensed by the means for measuring the temperature.
 9. The optical fiber polarization scrambler of claim 1, further comprising means for maintaining temperature of the birefringence modulator constant.
 10. An optical fiber polarization scrambler comprising: optical fiber birefringence modulators including at least two hollow cylindrical piezo devices and an optical fiber wound around outer walls of the hollow cylindrical devices; and alternating voltage sources applying alternating voltages to each optical fiber birefringence modulator to induce polarization modulation, wherein the polarization controller is inserted between the neighboring birefringence modulators, and wherein the hollow cylindrical piezo devices have different wall thickness to make the resonance frequency of the thickness mode of each piezo device of the optical fiber birefringence modulators different.
 11. An optical fiber polarization scrambler comprising: optical fiber birefringence modulators including at least two hollow cylindrical piezo devices and an optical fiber wound around outer walls of the hollow cylindrical devices; and alternating voltage sources applying alternating voltages to each optical fiber birefringence modulator to induce polarization modulation, wherein the polarization controller is inserted between the neighboring birefringence modulators, and means for maintaining temperature of the birefringence modulator; and means for determining modulation amplitude corresponding to the temperature sensed by the means for measuring the temperature.
 12. An optical fiber polarization scrambler comprising: optical fiber birefringence modulators including at least two hollow cylindrical piezo devices and an optical fiber wound around outer walls of the hollow cylindrical devices; and alternating voltage sources applying alternating voltages to each optical fiber birefringence modulator to induce polarization modulation, wherein the polarization controller is inserted between the neighboring birefringence modulators, and means for maintaining temperature of the birefringence modulator constant.
 13. An operating parameter input method for an optical fiber polarization scrambler comprising: optical fiber birefringence modulators including at least two hollow cylindrical piezo devices and a continuous optical fiber wound around each outer walls of the piezo devices; and alternating voltage sources for applying alternating voltage to each optical fiber birefringence modulator to induce polarization modulation; wherein said alternating voltage applied to each birefringence modulator is a sinusoidal waveform with different frequencies, and amplitude of the alternating voltage being configured as a value minimizing degree of polarization measured from output light signal sensed from an optical sensor after light from the light source passing in turn an input polarization controller, the optical fiber polarization scrambler, an output polarization controller and a polarizer.
 14. The operating parameter input method of claim 13, wherein thickness mode resonance frequency of the piezo device is used as the birefringence modulation frequency.
 15. The operating parameter input method of claim 13, wherein the birefringence modulation amplitude of each optical fiber birefringence modulator is selected from solutions of the zeroth-order Bessel function to operate the optical fiber polarization scrambler.
 16. An optical fiber polarization modulator having an optical fiber birefringence modulator and an alternating voltage source, said optical fiber birefringence modulator including a hollow cylindrical piezo device and a continuous optical fiber wound around outer wall of the piezo device, said alternating voltage source applying alternating voltage to the optical fiber birefringence modulator to induce birefringence modulation, wherein the frequency of the alternating voltage is adjusted to the proximity of the resonance frequency for thickness mode of the piezo device. 